Many processes for the breakdown of organic compounds in water by means of oxidation are generally known, just as are processes of adsorption of organics onto adsorbents, preferably activated carbon. In addition, processes for ultrafiltration or extraction are known.
Processes such as that of the present invention are customarily used in combination with a biological wastewater treatment when this concerns the elimination of poorly biodegradable or nonbiodegradable organic pollutants. In this Case, in Particular Oxidation Processes as wastewater pretreatment serve for the conversion of nonbiodegradable or poorly biodegradable compounds into compounds which are degradable by bacteria. A complete elimination of the organic compounds, that is to say from a high initial TOC (total organic carbon) of greater than 50 ppm, in particular from 100 to 2000 ppm, to TOC values of less than 10 ppm, on the basis of exclusively one of the abovementioned processes, or a combination of a plurality of the abovementioned processes, is not known.
EP-A 1 243 562 describes a combination of processes for eliminating organic compounds and nitrogenous organic compounds consisting of a first denitrification stage in the form of wet oxidation at 180-300° C. in a strongly acidic environment with the co-presence of a dinitrification agent in the form of a nitrate salt of an aliphatic or aromatic amine, and a second stage for TOC elimination. The latter can be adsorption, oxidation or neutralization and biological treatment. This produces a TOC elimination of 94%, or a TOC residual content of a maximum of 18 ppm.
A combination of important oxidation methods may be found, for example, in R. Munter et al., “Advanced Oxidation Processes (AOPs): Water Treatment Technology for the Twenty-first Century”, Kemia-Kemi Vol. 28 (2001) 5. In addition, therein the use of ozone, hydrogen peroxide in combination with one another and/or with UV irradiation and catalysts for generating hydroxyl radicals is described and they are compared with one another (theoretically). However, no indications can be obtained for the application of these processes to waters containing salt or additionally containing nitrogenous organic compounds.
In J. Fehn, K. Held, Wasser Abwasser, 136 (1995) No. 6 there is a description of ozonolysis, in particular in combination with H2O2, applied to poorly biodegradable industrial waters having a chloride content of 25 mg/l. Emphasis is given to the more efficient ozone consumption and the higher TOC degradation rate with elevation of the pH from neutral to pH 11.5. However, the initial TOC content was only 55 mg/l and nitrogenous organic compounds were not present. Reference is made to the interfering effect of hydrogencarbonate, likewise to the high capital costs and operating costs of ozonolysis.
In R. Hernandez et al., Journal of Hazardous Materials 92 (2002) 33-55, AOP methods such as UV, O3, UV/O3, UV/H2O2 and also H2O2/O3 on wastewater contaminated with 5 ppm of acetone are compared. Degradation of the ketone of 99% was only achieved with the combination of UV/O3, ozonolysis alone resulted in less than 40%. Nitrogenous organic compounds or salts were not additionally present.
An industrial ozonolysis procedure can be taken, for example, from U.S. Pat. No. 6,153,151.
WO 00/78682 describes a process for treating with ozone a wastewater from polycarbonate production containing more than 2 ppm of TOC and at least 0.1% by weight of dissolved carbonic acid or carbonates. The wastewater thus treated is said to be suitable for direct introduction into surface waters, but in particular, in the event that the wastewater contains dissolved common salt, to be suitable for reuse for producing chlorine by electrolysis. The TOC content of the water before the treatment is a maximum of 28 mg/l (comparative example), that is to say very low; the decisive organic compounds are phenols and amines. In contrast to the teaching from J. Fehn (see above) it is emphasized that in the neutral pH range significantly better TOC degradation takes place than at pH 12 (comparative examples). This is explained by the co-presence of carbonates. It cannot be gathered from WO 00/78682 whether high proportions of nitrogenous organic compounds, in particular from the group of azines and derivatives thereof, can also be completely eliminated by means of ozonolysis. Likewise there is no indication of residual nitrogen remaining which, as is generally known, must only occur in very low amounts of at most 10 ppm in electrolysis.
The prior art for use of common salt solutions in electrolysis processes is that the content of organic compounds and of organic/inorganic nitrogen must be so low that no impairment of the electrolysis itself and the service lives of the associated process components such as, for example, the membranes and/or the brine filtration and brine workup can occur. The common salt solution used in chlor-alkali electrolysis must therefore generally contain no more than 10 ppm of TOC when the diaphragm or amalgam electrolysis process is employed. When the membrane process is used, still lower TOC limiting values are required. The total nitrogen content must in all cases likewise be <10 ppm, in particular less than 1.0 ppm.
The production of chlorine and sodium hydroxide solution by electrolysis of common salt in aqueous solution can be gathered, for example, from Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 6, 5th Edition 1986, pp. 401-477. Extensive details can also be gathered from “Referenzdokument fiber die besten verfügbaren Techniken in der Chloralkaliindustrie” [Reference document on the best available techniques in the chlor-alkali industry], December 2001, Umweltbundesamt (German Federal Environmental Agency).
An object of the present invention was therefore the provision of a process as economical as possible for the substantially complete elimination of nitrogenous organic compounds from a salt-containing water having a high content of nitrogenous organic compounds before the treatment and also providing a salt-containing water resulting therefrom which is suitable not only for introduction into surface waters, but can also be fed to use of the dissolved salt for producing chlorine and/or sodium hydroxide solution by electrolytic processes.